Use of wind turbine power generating apparatuses has been promoted in recent years to preserve the environment, and a technique to connect a wind turbine power generating apparatus to a utility grid stably has been required.
Generally, to connect a wind turbine power generating apparatus to a utility grid to start power transmission, an anemometer disposed on an upper part of a wind turbine power generating apparatus (on a nacelle, for instance) firstly measures the wind speed around a wind turbine rotor, and if an average wind speed in a predetermined period exceeds a cut-in wind speed, the pitch angle of blades is changed from a feather side toward a fine side to start rotation of the wind turbine rotor, thereby starting the wind turbine power generating apparatus. When the rotation speed of the wind turbine rotor (or the rotation speed of a generator) reaches a connection rotation speed, the wind turbine power generating apparatus is connected to the utility grid.
For instance, Patent Document 1 discloses a method of changing the pitch angle of blades from the feather side toward the fine side at a constant rate, and matching a generator rotation speed with a connection rotation speed by adjusting the pitch angle if the generator rotation speed exceeds the connection rotation speed. Further, Patent Document 2 discloses a method of increasing the rotation speed of a wind turbine rotor by changing the pitch angle of blades from the feather side toward the fine side in stages before connection to a utility gird.